1. Field of the Invention
The present invention relates to an image reading apparatus for reading thick documents such as books and the like, and specifically relates to an image data correction method used in reading in a subscanning direction.
2. Description of the Related Art
Conventional apparatuses for reading documents require that documents such as books and the like are placed facing upward on a document platen and are read by a reading unit disposed above the document platen. In this type of apparatus, noise generated by illumination from external fluorescent lights and the like external to the apparatus is introduced between the document and the reading unit. The aforesaid noise produced illumination irregularities in the subscan direction occur because in general fluorescent lights and the like flash on and off at a commercial frequencies and their illuminance fluctuates over time. A shading reference panel is provided for correcting illuminance irregularities in the subscan direction, so as to measure the illuminance of said shading reference panel by a sensor and correct the read document data in accordance with the detection results (subscan shading correction).
For example, Japanese Unexamined Patent Application No. HEI 4-345271 discloses a method for shading correction wherein the read document data are corrected based on the read data of a shading reference panel of each one line in a subscan direction.
In conventional apparatus, however, the shading reference panel is flat, whereas a book-like document has a curved surface and, therefore, there are differences in the nature of the illuminance irregularities between the document surface and the shading reference panel which cause inadequate correction because luminance correction is accomplished only by the read data of the shading reference panel.
An object of the present invention is to eliminate the previously described disadvantages by providing an image forming apparatus capable of correcting illuminance irregularities in a subscan direction even when there is fluctuation of the elevation difference of the document surface and shading reference panel and the illumination irregularities of the two do not match.